1. Field of the Invention
The present invention relates to an engine block structure in a multi-cylinder engine, including a cylinder block and a cylinder head, with pistons being assembled into a plurality of cylinders provided in series in the cylinder block by inserting the pistons from the side of a crank chamber.
2. Description of the Related Art
A multi-cylinder engine is conventionally disclosed, for example, in Japanese Patent Application Laid-open No. 4-17759, in which an engine block is formed into a so-called mono-block structure including a cylinder block and a cylinder head which are formed by molding integrally with each other. A multi-cylinder engine is also disclosed, for example, in Japanese Patent Application Laid-open No. 5-26100, in which a cylinder head and a cylinder liner are formed integrally with each other in an engine block.
In such multi-cylinder engine including the cylinder block and the cylinder head formed integrally with each other, or including the cylinder head and the cylinder liner formed integrally with each other, the incorporation of the pistons into the plurality of cylinders is obliged to be carried out from a lower side of the cylinders, i.e., from the side of the crankcase for the structural reason. For this reason, the multi-cylinder engine is conventionally contrived, so that the assembling of the pistons into the cylinders can be carried out, while ensuring sufficiently a bearing area necessary for the crankshaft. The engine described in Japanese Patent Application Laid-open No. 4-17759, has a structure in which the crankcase is divided into an upper case portion (bulkhead) and a lower case portion, and a notch of semi-circular section for preventing an interference at the time of insertion of the pistons into the cylinders is provided in the upper case portion. The engine described in Japanese Patent Application Laid-open No. 5-26100 has a structure in which the distance between sides of a portion of the cylinder block in which a main bearing for the crankshaft is disposed, is equal to or slightly larger than an inside diameter of a cylinder liner-inserted portion. However, the former suffers from a problem that a crank journal portion is divided into upper and lower parts and for this reason, the number of parts is increased, but also the assemblability is degraded. The latter suffers from another problem that the distance between cylinder bores is increased, and an increase in length of the engine block in the direction of arrangement of the cylinders cannot be avoided, thereby bringing about increases in size and weight of the engine block.
The crankshaft supporting structure in the prior art multi-cylinder engine is generally formed such that joint surfaces of a plurality of journal walls at a lower portion of the cylinder block and a plurality of bearing caps corresponding to these walls are fastened to each other by bolts perpendicular to the joint surfaces, as disclosed in Japanese Patent Application Laid-open No. 4-17759. In such structure, however, there is a possibility that the bearing cap and the journal wall may be misaligned laterally (i.e., in a direction along the joint surfaces) from each other due to a machining or working error and/or an assembling error and further due to a variation in axial tension during hot after the assembling and/or the like.
Further, the prior art multi-cylinder engine has a structure in which a bolt coupling the joint surfaces of the journal wall and the bearing cap to each other is tightened from the lower side of the bearing cap. However, if this bolt can be tightened from the upper side of the engine it may provide such an advantage that the bolt tightening operation can be easily carried out without hindrance, even when the space below the bearing cap is insufficient. Especially, a multi-cylinder engine having a bearing cap-integral type lower block structure in which a plurality of bearing caps are formed integrally with an oil pan body, has a superiority that the oil pan body is not a hindrance to the bolt tightening operation. However, if the bolt is perpendicular to the joint surfaces as in the conventional structure, it is necessary, in order to ensure that the bolt can be tightened from above (i.e., from outside the cylinder block), to increase the size of the bearing cap into which the bolt is screwed. Especially, as for the bearing cap located at an end in the direction of arrangement of the cylinders, the following disadvantage is encountered: the amount of protrusion of this bearing cap in an axial direction of extension of the crankshaft is increased, resulting in an increase in size of the engine block in the axial direction of the crankshaft.